The present invention relates generally to optical networks, and in particular to a method and system for determining location and value of dispersion compensating modules (DCMs) in optical networks.
As development of long-haul networks begins to saturate and the demand for larger bandwidth in the networks starts to expose bottlenecks at the user-end, the focus of optical network development has shifted away from long-haul networks to smaller and more dynamic networks, for example metropolitan area networks (MANs). As a result, new network design and planning rules, for example related to the placement of network components, have to be developed.
Specifically, determining the location and value of dispersion compensating modules (DCMS) in MANs is a new design issue that has arisen since the growth in the size of MANs has reached the degree that dispersion compensation has become necessary in MANs. However, many of the current DCM design methods were developed for long-haul networks and may not always be appropriate for MANs.
For example, long-haul networks are subject to fiber nonlinear effects due to high signal power levels required for transmission over long spans of fiber, and DCMs are often used to counter these nonlinear effects while eliminating chromatic dispersion, as illustrated by the following two patent documents. U.S. Pat. No. 5,559,920 to Chraplyvy et al. issued Sep. 24, 1996 and entitled xe2x80x9cDispersion Compensation in Optical Fiber Communicationsxe2x80x9d, discloses a method of placing dispersion compensating fiber (DCF) in a long-haul network whereby the dispersion of the signals in the network is always limited to the linear regime by implementing xe2x80x9cdispersion averagingxe2x80x9d. Another U.S. patent application 20020093706 to Lu et al. dated Jul. 18, 2002 and entitled xe2x80x9cDispersion Managed Optical Transmission for Wavelength Division Multiplexed Systemsxe2x80x9d discloses a method of selecting DCM values to minimize nonlinear effects such as self-phase modulation, inter-symbol interference, and cross-phase modulation, while minimizing chromatic dispersion.
MANs, on the other hand, have lower signal power levels than long-haul networks so that fiber nonlinearity is typically not an issue, and therefore it is not always necessary to attempt to minimize non-linear effects when determining the location and value of DCMs in an optical MAN.
In the above-mentioned patent documents, as in most methods for DCM design of long-haul networks, the dispersion compensation is performed on a per-span basis so that a DCM is placed on each fiber span in the network. In contrast, it is inefficient and costly for DCMs to be placed on every fiber span in a MAN, and because MANs are more cost-sensitive than long-haul networks, minimizing the total number of DCMs in the network is the motivating factor in DCM design methods for MANs.
Currently, there are existing methods for determining the location and value of DCMs in a MAN whereby DCMs are placed on only a selected number of fiber spans in the network. These methods are performed manually using the intuition and experience of a designer.
An optical university project by L. Chrotowski, C. Mateus, F. Mo, and L. Zhou at the University of California, Berkeley dated Dec. 17, 2001 and entitled xe2x80x9cOptical Network Design of a Metro Ringxe2x80x9d discloses a method of DCM design in a metro ring network involving quantifying the degree of eye closure on a signal, which is used as the factor upon which the DCM placement is dependent. The placement itself however is determined heuristically by the designers, who attempt to minimize the total number of DCMs in the network by determining placement of just enough DCMs so that the network is operating within desired conditions (in this case to a maximum value of eye closure).
However, real-world MANs are topologically complex and often take the form of rings or meshes that may include coupled lightpaths. With increasing size and complexity of MANs, the current manual, heuristic methods of DCM placement become impractical and inefficient.
Therefore, there is a need in the industry for the development of a systematic method and system for determining the location and value of DCMs in an optical network that would be efficient and applicable to a variety of network topologies.
Therefore there is an object of the invention to provide a method and system for determining the location and value of DCMs in an optical network that would avoid or minimize the above-mentioned drawbacks.
According to one aspect of the invention, there is provided a method for determining the location and value of one or more DCMs in an optical network, comprising the steps of:
(a) determining a lightpath topology in the network;
(b) determining an effective dispersion on each of the lightpaths in the network, the effective dispersion being an amount of dispersion accumulated along the lightpath which exceeds the maximum positive dispersion value Pos_Disp_Limit specified for the network;
(c) if the effective dispersion is positive, calculating an effectiveness score for each combination of a DCM value and location in the network, the effectiveness score being a measure of dispersion compensation by the DCM over one or more lightpaths that pass through the DCM;
(d) selecting the DCM location and value combination based on said effectiveness scores; and
(e) repeating the steps (b) to (d) until the effective dispersion in the network is substantially zero.
Conveniently, the step of determining the lightpath topology comprises identifying all lightpaths in the network including protection lightpaths and reconfigurable lightpaths.
In the method of the first embodiment of the invention, the step (d) of selecting the DCM location and value combination may comprise selecting the DCM location and value combination whose DCM calculation object has the highest effectiveness score.
The step (b) of determining the effective dispersion may comprise measuring the effective dispersion in units of distance.
The step (a) of determining the lightpath topology may comprise:
identifying lightpaths in the network;
assigning lightpath identification numbers (lightpath IDs) to the lightpaths;
identifying fiber spans over which the lightpaths are laid.
Furthermore, the step (c) of calculating the effectiveness score may comprise summing the effectiveness scores of each lightpath that passes through the DCM, wherein the effectiveness score for a lightpath is equal to:
the effective dispersion on the lightpath, if the effective dispersion is less than the DCM value; and
the DCM value, if the effective dispersion on the lightpath is greater than the DCM value.
Also, the method may further comprise the step of introducing a DCM calculation object and initializing the values thereof for each combination of a DCM value and location, the calculation object including the following components:
lightpath identification numbers (lightpath IDs) of lightpaths that pass through the DCM;
a validity status, indicating the permission for placing the DCM in the specified location in the network;
an effectiveness score, providing a measure of dispersion compensation by the DCM over one or more lightpaths that pass through the DCM.
Additionally, the step of calculating the effectiveness score may comprise:
(i) selecting the DCM calculation object;
(ii) verifying if the accumulated dispersion along a lightpath that passes through the DCM is above the maximum negative dispersion value Neg_Disp_Limit specified for the network;
(iii) if no, updating the validity status to be xe2x80x9cinvalidxe2x80x9d;
(iv) if yes, re-calculating the effectiveness score by taking into account the values and locations of the selected DCMs; and
(v) repeating the steps (i) to (iv) until all DCM calculation objects are selected.
The step of introducing the DCM calculation object may comprise defining the validity status as a Boolean variable having values of xe2x80x9cvalidxe2x80x9d and xe2x80x9cinvalidxe2x80x9d and indicating permission for placing the DCM in the specified location in the network, the Boolean variable being xe2x80x9cvalidxe2x80x9d if the accumulated dispersion along each of the lightpaths passing through the DCM is above the maximum negative dispersion value Neg_Disp_Limit specified for the network.
Also, the step of introducing the DCM calculation object may comprise defining the validity status as the number of remaining DCMs permitted to be placed at the DCM location.
In a modification to the method of the first embodiment of the invention, the step (c) of calculating the effectiveness score comprises calculating the effectiveness score by taking into account a dispersion slope mismatch between the dispersion of fiber spans in the network and the dispersion of the DCM.
The step of calculating the effectiveness score by taking into account a dispersion slope mismatch may comprise calculating the effectiveness score as equal to the minimum compensated dispersion over all wavelengths for lightpaths that pass through the DCM.
In another modification to the method of the first embodiment of the invention, the step (d) of selecting the DCM location and value combination further comprises determining alternative DCM locations such that the transfer of the DCM to the alternative location does not change the accumulated dispersion along any lightpath in the network.
According to another aspect of the invention, there is provided a method for determining the location and value of one or more dispersion compensating modules (DCMs) in an optical network, comprising the steps of:
(g) executing the method as described in claim 1, wherein the step (d) of selecting the DCM location and value combination comprises selecting the DCM location and value combination whose DCM calculation object is chosen from the DCM calculation objects in the network;
(h) repeating the step (g) required number of times; and
(k) comparing the total numbers and values of DCMs provided by methods executed in steps (g) to (h); and
(l) choosing the method among methods executed in steps (g) to (h) that provides the least number of DCMs and the lowest value of DCMs in the network.
The method may further comprise saving all calculated values. Also, the step (h) may comprise repeating the step (g) for the DCM calculation object, which has the highest effectiveness score.
Additionally, the step (g) of executing the method may comprise selecting the DCM location and value combination whose DCM calculation object is chosen from a subset of the DCM calculation objects. Beneficially, the subset of DCM calculation objects may be the subset with the highest effectiveness scores.
Furthermore, the step (d) of selecting the DCM value and location combination may further comprise verifying that the total number of selected DCM value and location combinations is less than that of previous executions of the step (g), otherwise terminating the execution of the step (g).
According to yet another aspect of the invention, there is provided a system for determining the location and value of one or more dispersion compensating modules (DCMs) in an optical network, comprising:
(a) means for determining a lightpath topology in the network,
(b) means for determining an effective dispersion on each of the lightpaths in the network, the effective dispersion being an amount of dispersion accumulated along the lightpath, which exceeds the maximum positive dispersion value Pos_Disp_Limit specified for the network;
(c) if the effective dispersion is positive, means for calculating an effectiveness score for each combination of a DCM value and location in the network, the effectiveness score being a measure of dispersion compensation by the DCM over one or more lightpaths that pass through the DCM;
(d) means for selecting the DCM location and value combination based on said effectiveness scores; and
(e) means for checking that the effective dispersion in the network is substantially zero.
In a modification to the first embodiment of the invention, the means (c) for calculating the effectiveness score comprises means for calculating the effectiveness score that takes into account a dispersion slope mismatch between the dispersion of fiber spans in the network and the dispersion of the DCM.
In another modification to the first embodiment of the invention, the means (d) for selecting the DCM location and value combination further comprises means for determining alternative DCM locations such that the transfer of the DCM to the alternative location does not change the accumulated dispersion along any lightpath in the network.
The methods for determining the location and value of DCMs in an optical network of the embodiments of the invention provide a systematic procedure that is efficient and applicable to a variety of network topologies.